A sodium-sulfur battery may be given as one example of electric power storage devices which are expected to be used more and more in various application fields, such as electric-load leveling, measures against a momentary drop in power, or compensation for fluctuations in the power generated by a natural energy generating system.
Such a sodium-sulfur battery is generally configured in such a manner that a plurality of electric cells is connected in series to form a string, a plurality of thus connected strings is connected in parallel to form a block, a plurality of thus connected blocks is connected in series to form a module, and finally a plurality of thus connected modules is connected in series. And, the operating cycle of the sodium-sulfur battery, in case of the application of the load leveling, for example, is a repetition of a cycle consisting of a discharge and a charge with an intervening idle period there between.
Failure of the sodium-sulfur battery to exhibit its expected performance would interfere with the aforesaid applications, such as the load leveling, measures against a momentary drop in power, and the compensation for fluctuations in power. Further, if the sodium-sulfur battery repeats charges and discharges, deviating from its set operational range, then the battery will be overdischarged or overcharged, leading to reduction in the performance thereof. It is important, therefore, to accurately grasp the state of charge of the sodium-sulfur battery.
As one of the conventional methods for calculating the state of charge of a sodium-sulfur battery, there has been known, for example, a method for calculating the state of charge by integrating the charge currents or the discharge currents of the sodium-sulfur battery and then calculating the state of charge on the basis of the integrated value and a rated capacity thereof. More specifically, the integrating value of the discharge currents may be subtracted from the rated capacity at the time of a discharge, while the integrated value of the charge currents may be added to the state of charge (before the charge) to calculate a (an updated) state of charge at the time of a charge. JP-A-08-017478 may be given as an example of a prior art.